1. Field of the Invention
The present invention relates to Al alloy films and melting Al alloy sputtering targets for depositing the thin films, and particularly, to Al alloy films used as (1) shading films for liquid crystal display panels, solid state pickup devices (elements), etc., (2) reflection films for optical recording media such as magneto-optical disks, (3) Al alloy thin film line or electrode materials for semiconductor devices, and to (4) melting Al alloy sputtering targets for depositing the thin films.
2. Prior Art
(1) Shading Films
(1)-(a) Recently, there has been demanded a liquid crystal display panel with a large sized screen and high resolution. To meet the demand, active matrix system using 2-terminal or 3-terminal elements are in the development. Among them, those using thin film transistors (hereinafter referred to as TFT) have been mainly used as the large sized and high resolution liquid crystal display panels.
Such a liquid crystal display panel is obtained by partially forming a semiconductor area of a-Si, p-Si, etc. on a transparent insulating substrate of quartz, glass, etc., forming TFT's serving as switches inside the area, and forming display bodies such as electrodes and liquid crystals used as a display panel over them. In the above, the transparent substrate is disadvantageous in that the rays of light come into the semiconductor area, and a photo-excited current flows, which makes OFF operation insufficient. As a countermeasure for preventing the light from coming into the semiconductor area, there is used a method for depositing metal films (shading films) on the upper surface, on the lower surface or on both surfaces of the TFT area to shield the light. Such shading films are used not only for shielding the semiconductor area, but also for upgrading contrast by depositing the films into grid shaped narrow strips on the upper and lower sides of areas between respective picture elements.
Refractory metals such as Cr, etc. or colored resins have been used as the shading films. However, in such shading films for liquid crystal display panels, that is, shading films of a refractory metal or a colored resin, the reflectance is low so that the light is absorbed in the shading films, resulting in the increased temperature in the shading films. Particularly, in the metal films having high thermal conductivity, temperature is easily raised inside the light-shielded area (the surface close to a liquid crystal area inside the shading films) by the light irradiated on the outer surface of the shading films, so that the liquid crystal temperature is raised, an electrochemical reaction in the electrode interface of the liquid crystal is expedited and the TFT OFF current is increased, thereby causing a remarkable degradation in display qualities.
As a countermeasure therefor, high reflective shading films are used in place of the above-mentioned low reflective shading films. Au, Cu and Al are suitable for depositing the high reflective shading films, and particularly, Al is most suitable therefor because of inexpensiveness, excellent adhesion to a substrate and facility for etching. Therefore, there have been generally used pure Al films or Al--Si alloy films (Si content is small). However, such pure Al films or Al--Si alloy films are disadvantageous in that there occur hillocks due to the residual stress generated in the films during a depositing process or a laminating process, or due to heat history generated in manufacturing the liquid crystal display panel. Also, they have a tendency to easily produce pin holes so that, at the time of irradiation of the light, the light through the pin holes activates a TFT to thereby generate an erroneous electric signal. Furthermore, they are disadvantageous in that the temperature is raised at the time of irradiation of the light due to the high thermal conductivity, thereby degrading the display quality of the liquid crystal panel. Particularly, in the projection type liquid crystal display capable of displaying a large sized picture, which is developed actively in recent years, a strong light is irradiated by a halogen lamp, etc. in order to secure brightness, which makes the generations of erroneous electric signals and hillocks conspicuous.
(1)-(b) With the rapid progress of the image pickup ability, the solid state image pickup device (element) has been strongly required to enhance its performances furthermore. A sectional view of a conventional solid state image pickup element is shown in FIG. 17. Shading films 5 having light receiving windows in a matrix shape are deposited on a semiconductor substrate 1. The shading films 5 and a signal line 7 are often formed in the same process, so that the pure Al films or Al--Si alloy films have been generally used. In FIG. 17, reference numerals 2 is an N-type diffusion layer, 3 is an electrode, 4 is an insulating film between layers and 6 is an insulating film. A P-type silicon substrate is commonly used for the substrate 1.
However, the conventional shading films for the solid state image pickup element, that is, the pure Al films or Al--Si alloy films are disadvantageous in that heat transfer is generated in Al by the annealing (300 to 400.degree. C.) for eliminating the surface level of the semiconductor substrate, to thereby generate hillocks on the shading films. When a hillock is protruded from the edge of a light receiving window toward the light receiving element side, the light receiving area of the light receiving element is made smaller than others' so that the sensitivity is lowered. Another disadvantage is to easily produce pin holes resulting in the erroneous electric signal.
(2) Reflection Films
High reflectance is required for reflection films on a substrate constituting a principal area of an optical recording medium such as an optical disk or a magneto-optical disk, so that pure Al films have been generally used. The pure Al films, however, are poor in corrosion resistance and have a disadvantage of being corroded when placed in air for a long time, to reduce the reflectance or generate pitting corrosion, thereby increasing the read error rate in the playback of information (signal).
As a countermeasure therefor, there have been proposed Al alloy films having various compositions, for example, Al alloy films containing Si, Pt or Pd. The proposed alloy films, however, are still insufficient in corrosion resistance to secure the long term reliability in the reproduction of information records in optical recording media. Further, at the film deposition, it is necessary to secure the homogeneity of alloy composition of the films. In order to achieve this purpose, a sputtering process is superior to a vapor deposition process. In the sputtering process, however, the film deposition rate is slow and it takes a longer time for film deposition in comparison with a vapor deposition process, which lowers the throughput in a film deposition process in mass production, and it can be an obstacle for improving the productivity. Therefore, the conventional Al alloy films have a disadvantage of requiring a long time for the film deposition resulting in the reduced productivity.
(3) Al Alloy Thin Film Line and Electrode Material for Semiconductor Devices
(3)-(a) Al alloy thin films are used as the materials for the electrodes and lines of integrated circuits for general semiconductor devices (a semiconductor device in which elements are formed on a Si wafer), and which are divided broadly into two categories, pure Al and Al based alloys containing Si or Cu.
Pure Al is the most excellent material in the viewpoint of low electric resistivity. However, the pure Al has disadvantage of generating stress-migration (hereinafter referred to as SM) or electro-migration (hereinafter referred to as EM). SM is the swelling (hillock) and the disconnection (bad conduction) of the film line caused by stress, and which is mainly generated by heating. EM is the disconnection of film line caused by electric migrations, and which is mainly generated by current-carrying.
Al alloys containing Si or Cu have been developed for improving the above-mentioned disadvantages, but they are not sufficient in the resistance against SM and EM, and also in corrosion resistance. Accordingly, there has been desired the development of novel materials having higher reliability for semiconductor devices.
To obtain the novel material, further alloying can be considered. However, the alloying tends to increase in general the resistivity. On the other hand, the allowable upper limit of resistivity tends to be lowered by the miniaturization of line (miniaturization in width) accompanied with the higher degree of circuit integration in the recent semiconductor devices, and simple alloying exceeds the above-mentioned upper limit. Consequently, the development of a novel material for semiconductor devices are in a difficult situation.
(3)-(b) The materials for electrodes and lines of a TFT used for liquid crystal displays, etc. are, differently from the materials for the above-mentioned general semiconductor devices, heated up to a comparatively high temperature (about 400.degree. C. or below 400.degree. C.) in a manufacturing process of a TFT, so that Al based metal materials are poor in SM resistance. Consequently, refractory materials such as Ti, Cr, Mo, Ta, etc. are often used. However, there is a disadvantage that the resistivity is high, and the improvement is desired.
Recently, liquid crystal displays are getting to be larger in sizes for displaying larger pictures, and the line connecting between TFT elements (address line) is liable to be lengthened, and the resistance and capacitance are increased therewith so as to easily cause the delay of address pulses. As a result, the above-mentioned refractory metal materials are difficult to be used, and a novel refractory metal material having low resistivity is desired to be developed.
The resistivity of such line is desired to be lower than about 30 .mu..OMEGA.cm, and the metals, Au, Cu and Al, satisfy the above-mentioned value. Among these metals, Au is difficult to be used because of expensiveness, Cu has a disadvantage in adhesion and in corrosion resistance. Al has a disadvantage in low melting point and in poor heat resistance, and is liable to occur the short circuits between layers (or between lines) or disconnections by SM. Accordingly, no one among them can be practically used.
Then, multilayer line (composite line), alloying of surface areas of line by ion implantation (surface alloyed line), etc. have been proposed. The multilayer line includes both a lower layer of a low resistance material and an upper layer of a high refractory material, to achieve a composite function of a lower layer (low resistivity) and an upper layer (SM resistance). The surface alloyed line is provided with the refractory layer on the surface area by ion-implantation of different elements into a low resistance material, to thus achieve a similar function to the above-mentioned multilayer line.
In the multilayer line, 2 times of film deposition processes must be performed, and in some combination of the upper and lower layers, the etching for forming a line pattern must be performed in a different process, to thereby increase the number of processes and to reduce the productivity. Meanwhile, the surface alloyed line needs a complicated process of ion implantation and is difficult in control of a surface alloyed layer, to thereby increase the number of processes and to reduce the productivity. Accordingly, there is desired the development of a novel metal material for a TFT which has no such disadvantages as mentioned above, that is, having low resistivity and being excellent in refractory.
(4) Sputtering Targets for Depositing Al Alloy Films
From the viewpoint of homogeneity of the alloy composition of films, the above-mentioned Al alloy films is preferably deposited by a sputtering process in comparison with a vapor deposition process. In the sputtering process, following sputtering targets have been used: (a) a small piece of a metal (alloy component) such as Cr is placed on a pure Al target; (b) blocks of a pure Al and alloy component are disposed in a mosaic pattern; and (c) powder of a pure Al and alloy component are mixed and sintered.
In the cases of (a) and (b), since there is a difference in the sputtering yield and the radiation angle between an Al and alloy component, the composition of the obtained Al alloy films becomes smaller than that of the target (area ratio between an Al and alloy component), and the difference varies according to the conditions of sputtering or to the devices. Therefore, it is difficult to control the composition of Al alloy films, and since the above-mentioned area ratio varies continuously during the usage of a target material, Al alloy films of a specified composition is difficult to be obtained. In the case of (c), since an Al powder comparatively differs from an alloy component powder in the specific gravity, it is difficult to mix them homogeneously and hence to obtain the homogeneous composition of a target material, so that the Al alloy film composition is liable to be inhomogeneous. Furthermore, the above-mentioned both powders are active and tend to absorb oxygen strongly, so that the Al alloy films contain much oxygen thereby degrading the reflectance and also increasing the electric resistivity.